Balanced-detection interferometric cavity-assisted photothermal spectroscopy employing an all-fiber-coupled probe...

The interferometric cavity-assisted photothermal spectroscopy (ICAPS) method has been proven highly suitable for sensitive and compact gas detection by application of an optical cavity as transducer for photothermal spectroscopy. This work reports on the implementation of an overall fiber-coupled probe laser configuration detecting the reflectance of the individual interferometers in a balanced-detection ICAPS system. The layout greatly improves the overall sensor system robustness. Two identical 1 mm path length cavities were used for balanced detection, enabling sensor operation close to the fundamental limit of shot noise by efficiently cancelling excess noise. A quantum cascade laser served as a mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced refractive index variations. The metrological figures of merit for the sensor were investigated by SO₂ detection. For the targeted absorption band centered at 1380.93 cm^-1, a 3 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 4.5 × 10^-9 cm^-1 W Hz^-1/2.